This invention concerns a test method for phosphate coatings on iron based surfaces.
Conventionally phosphate coatings have been widely used industrially as an undercoating for paint in order to improve the adhesion of the paint and to improve corrosion resistance. It is known that the weight of the phosphate coating and the proportions of hopeite (Zn.sub.3 (PO.sub.4).sub.2.4H.sub.2 O) and phosphophillite (Zn.sub.2 Fe (PO.sub.4).sub.2.4H.sub.2 O) in the coatings (this ratio is referred to below as P/P+H) have an effect on the adhesion of the paint and on the corrosion resistance, and these are important characteristics which determine the efficiency of a phosphate coating.
Conventional methods for determining the weight of a coating are shown in Table 1.
TABLE 1 ______________________________________ Measur- Sample Method of Type of ing Sample Sec- Measurement Test Time Area Equipment tioned ______________________________________ x-ray non- 20 1-5 x-ray Yes fluores- destruc- seconds cm.sup.2 fluores- cence tive cence analyzer Stripping Destruc- 30 50-200 Direct Yes with aqueous tive minutes cm.sup.2 indicat- chromic acid ing balance ______________________________________
It is clear from Table 1 that determination of the weight by means of X-ray fluorescence has the advantage of permitting measurements to be made in a short period of time, but the equipment is expensive for a production line and there is a disadvantage in that the sample has to be sectioned. Furthermore, the method of stripping with aqueous chromic acid has been used conventionally for determining the weight of a coating but the stripping is carried out at a high temperature and takes a long time and there is a further disadvantage in that a chemical treatment must be carried out with the waste because of the presence of hexavalent chromium.
The conventional method for determining the value of P/P+H for a phosphate coating involves the use of X-ray diffraction. However, measurements cannot be made with this method while the object is at a high temperature after being phosphate treated and the equipment is expensive and it is not practical for a production line. There are other methods besides X-ray diffraction analysis in which the zinc ion and iron ion concentrations in an aqueous solution obtained by stripping with aqueous chromic acid are determined but this has the same disadvantages as the aforementioned method for determining the weight of the coating using an aqueous chromic acid stripping liquor. Furthermore, the zinc ion and iron ion concentrations cannot be determined colorimetrically because of the high chromic acid content. The analysis is normally carried out using atomic absorption spectroscopy, but the equipment is expensive and this is not practical as a production line method. Furthermore, if the P/P+H value obtained by X-ray diffraction is taken to be 100%, then the result obtained with the stripping method is quite low at 84% and this is undesirable.
The aim of this invention is to provide a test method for phosphate coatings which is free from these disadvantages.